1. Field of the Invention
The invention is related generally to blast furnace equipment as utilized in the manufacture of steel and, more specifically, to the construction of side-combustion hot blast stoves used to preheat the blast before introduction of that blast into a blast furnace.
2. Background of the Prior Art
The conventional construction and operation of side-combustion hot blast stoves is aptly set forth in "The Making, Shaping and Treating of Steel", 1971 edition, published by U.S. Steel Corporation, at pages 439-441. Included in this treatise are general diagrams of conventional hot blast stove design and construction.
The function of the breast wall in a hot blast stove is to separate and insulate the side-combustion chamber from the refractory checker chamber. The breast wall must be structurally sound to support the side-combustion chamber so as to maintain the refractory checkers free of lateral stress. In addition, the breast wall is required to be gas tight to prevent lateral leakage back and forth between the combustion chamber and the checker chamber.
Older designs of checkers utilized rather rugged thick sections for the checker cross section. This thick-section type of design necessarily reduced the overall area of refractory available in the checker chamber for heat transfer. However, the thick sections were deemed necessary to counteract the lateral forces directed on the checkers from the expansion of the breast wall when the hot blast stove was in the on-gas mode and combustion was taking place in the combustion chamber. The combustion produces heat which causes the face of the breast wall adjacent the combustion chamber to expand at a rate greater than the expansion of the opposite wall and the adjoining checker chamber. This expansion tends to produce a lateral crushing force on the checker brick in the checker chamber.
A further problem caused by the differential expansion of the breast wall develops from the fact that conventional breast walls are constructed of individual refractory shapes or brick. When the differential expansion takes place, the joints between these brick open up producing passageway through which hot gases can seep, thus derogating from the designed gas passage upwards through the combustion chamber to the dome, then downward through the checker chamber. Such gas seepage provokes localized hot spots at random points in the checker chamber adjacent the breast wall. These hot spots create rapid deterioration of the checker brick at those points and also tend to produce uneven heating, thus unbalanced cooling stresses in the checker brick when the hot blast stove is switched to the on-blast mode and the combustion heat is regenerated. In addition, highly localized overheating can occur on the metallic support structures related to the checker chamber resulting in distortion and/or failure thereof.
The design of the checker chamber has progressed, in recent years, in the direction of greater efficiency of heat transfer. Theoretically, it is well known that the more surface area available to preheat the cold gas being drawn through those checkers, the higher the average heat of the hot blast output from the stove. If the hot blast from the stove can be maintained at a higher average temperature, i.e., less of an amount of cold blast being required in the mixture to produce a uniform temperature, then the more efficient can be the operation of the blast furnace to which the hotter blast is being fed.
Practically, the increases in surface area within the checker chamber are limited by the requirement that the structural integrity of the checker brick must be maintained. It is a well-known axiom that the greater amount of surface area exposed in a checker brick, the thinner the wall sections of the checker must be. But this axiom must be limited in practice by the physical limitations applicable to the checkers. Checker brick wall or section thickness cannot be reduced below certain limitations because of the lateral crush forces that are imposed on the checker brick in operation in the checker chamber. Thus, in known design techniques the maximum heat transfer area available within a checker chamber of a particular size is limited by the lateral stresses that will be imposed by the differential expansion of the breast wall.
Vertical arch forms for hot blast stove breast walls have long ago been tried as such forms appeared to provide a rather simplified structure which was relatively less costly to construct and maintain as well as providing space economy. However, it was found that the conventional arch pattern, formed with regular key-shaped refractory brick, increased beyond tolerable limits the lateral stress imposed onto the checker brick within the checker chamber. Rapid deterioration of the checker brick resulted with a commensurate reduction in the ability of the hot blast stove to preheat the blast. Premature rebuilds of the hot blast stoves were required which were costly. Thus, the use of breast walls formed by a conventional vertical arch was rapidly abandoned in favor of forms including a modified arch similar to those shown in the above reference to "The Making, Shaping and Treating of Steel".
The main object of the present invention is to provide for the simplicity and economy of a vertical arch in hot blast stove breast wall construction, while substantially eliminating the lateral crush stress on the checker brick in the checker chamber, when the hot blast stove is in the on-gas mode, as caused by the differential expansion of the breast wall. Another object of the invention is to substantially reduce gas passage from the combustion chamber through the breast wall into the checker chamber when the hot blast stove is in the on-gas mode as well as the elimination of short circuiting of air flowing from the checker chamber to the combustion chamber during the on-blast mode.